Chemical mechanical polishing endpoint detection by monitoring component activity in effluent slurry

ABSTRACT

A method for determining the endpoint in a chemical mechanical polishing operation used for polishing a metal-containing material. An acidic polishing slurry is used to oxidize the metal and the oxidized metal is included in an effluent slurry stream. The effluent slurry stream is directed into a vessel which forms an electrochemical cell. The component activity of the effluent slurry stream is monitored within the electrochemical cell by measuring the electric potential across the electrodes of the electrochemical cell. When the measured electric potential changes, indicating a change in the composition of the effluent slurry, endpoint is indicated.

FIELD OF THE INVENTION

The present invention relates generally to chemical mechanical polishingof substrates, and more particularly to a method for detecting apolishing endpoint by monitoring the component activity in effluentslurry of the polishing operation.

BACKGROUND OF THE INVENTION

Chemical mechanical polishing (CMP) is one method of providing aplanarized substrate surface. Such substrates are used in themanufacture of integrated circuit devices. CMP may be used to planarizeraw substrates or to completely or partially remove a bulk depositedlayer, but is more commonly used to planarize a surface by partiallyremoving layers which have been deposited over non-planar featuresformed in, or on, a subjacent layer. A typical CMP apparatus employs arotating polishing surface, such as a consumable polishing pad, againstwhich the surface of the substrate being polished, is placed. The CMPapparatus also includes a carrier which secures the substrate in adesired position with respect to the pad. The carrier includes means forproviding a force to keep the substrate in contact with the pad, and mayalso include means for rotating, vibrating, or oscillating thesubstrate. During polishing, a slurry having both chemical and abrasiveagents is supplied to the interface between the substrate and the pad toenhance the rate at which material is removed from the substrate. Thechemical agents included in the slurry are generally chosen to bereactive towards the material being removed by polishing. The effluentslurry therefore includes products containing components of the materialremoved by polishing.

One problem associated with CMP is endpoint detection. Endpoint may bedefined as the point at which the desired polishing operation iscompleted. When “endpoint” is attained, a number of different actionsmay be taken in response. For example, the entire polishing process maybe terminated when endpoint is attained or the polishing conditions maybe changed as the polishing process continues with another polishingoperation, to polish an underlying film. It can be seen that a substratecontaining a stack of films to be polished, may include a number ofdiscrete polishing operations, each of which includes an associated“endpoint”.

Depending on the chemical mechanical polishing operation beingperformed, “endpoint” may signify different events. For example, whenpolishing a raw substrate, “endpoint” may be attained when a certainpredetermined substrate thickness has been removed. The same is true fora layer or film which is being partially removed. When a film is beingcompletely removed from a substrate, “endpoint” is attained uponcomplete removal of the film. When CMP is used to planarize a substrateby removing portions of a film which extend above underlying features,“endpoint” is attained when the surface is essentially planar. Generallyspeaking, an endpoint condition is attained after a predictable amountof material has been removed from the surface. It is therefore necessaryto accurately detect when endpoint is reached so that the polishingoperation may be quickly terminated or otherwise adjusted at that point.Because the substrate is polished face-down and the polishing surface isgenerally contiguous with the polishing pad, a process monitor cannoteasily be used to view the progress of the polishing operation,especially by directly monitoring the surface being polished. As such,it is difficult to attempt to use such a monitor to determine thepolishing “endpoint.”

Variations in the polishing conditions also impede an accuratedetermination of the polishing endpoint. For example, variations in theslurry composition and flow rate, pad condition, relative speed betweenthe pad and the substrate, the material being polished, and the load ofthe substrate on the pad, cause variations in the material removal rate.These variations in the material removal rate cause variations in thetime needed to reach the polishing endpoint. Therefore, the polishingendpoint cannot reliably be estimated merely as a function of polishingtime.

A common object of CMP is to planarize a substrate surface usingdamascene technology. In damascene technology, trenches, grooves orother openings may be formed within a subjacent layer such as adielectric film formed over a substrate. Next, a bulk deposited layer,generally a conductive film such as metal, is formed over the uppersurface of the subjacent layer and within the openings which extend downinto the subjacent layer. One aspect of CMP is to remove the bulk of thedeposited conductive layer from over the plane formed by the uppersurface of the subjacent layer, leaving areas of the conductive layeronly in the openings formed within the subjacent layer. In this manner,a wiring pattern is produced. It can be understood that it is desirableto terminate the polishing operation when endpoint is attained, i.e.when the bulk of the deposited conductive film is removed from over theplane formed by the upper surface of the subjacent layer, but remainswithin the openings so that the remaining portions of the conductivefilm form a substantially planar surface with the upper surface of thesubjacent layer.

One general approach to predicting the polishing endpoint is to removethe substrate from the polishing apparatus and measure the thickness ofthe substrate or the film being removed by polishing. By periodicallyremoving the substrate from the polishing apparatus and measuring itsthickness, the quantity of material being removed from the substrate maybe determined. As such, a linear approximation of the material removalrate may be used to determine the polishing endpoint. This technique istime consuming, however, and does not account for sudden changes in theremoval rate that may occur between measurement intervals, or for othervariations in the material removal rate as discussed above.

Several other non-invasive techniques for endpoint detection are known.These techniques generally fall into two categories: those which requireaccess to the surface of the substrate being polished, and those whichdetermine the polishing endpoint by determining changes in the operatingconditions of the polishing apparatus.

Techniques included within the first category typically requirereal-time access to at least a portion of the substrate surface beingpolished, such as by sliding a portion of the substrate over the edge ofthe polishing pad and simultaneously analyzing the exposed portion ofthe substrate. For example, where polishing is used to remove the bulkof a metal film, and to form metal lines embedded within trenches formedin a subjacent dielectric layer as in the planarization examplediscussed above, the overall or composite reflectivity of the surfacebeing polished changes as the bulk metal film is removed and thedielectric layer is exposed. By monitoring the reflectivity of thepolished surface or the wavelength of light reflected from the surface,the polishing endpoint can be detected as the reflectivity changes whenthe dielectric layer is exposed. However, this technique does notprovide a way of determining the polishing endpoint unless an underlyinglayer such as the dielectric is exposed during polishing and has areflectivity which varies from that of the film being removed bypolishing. Additionally, it is somewhat erratic in predicting thepolishing endpoint unless all of the underlying surface of a differentreflectivity, is simultaneously exposed. Furthermore, the detectionapparatus is delicate and subject to frequent breakdown caused by theexposure of the measuring or detecting apparatus to the polishingslurry.

Another technique included within first category involves projecting alaser beam through an opening formed in the polishing pad, and onto thesurface being polished. This technique is not favored because of thedifficulty associated with projecting a laser through an opening whichmust be formed in an otherwise uninterrupted, rotating polishing pad.Additionally, the window, through which the laser beam is projected,must be kept clean. This is quite difficult to do, especially with somecommonly used polishing slurries.

Techniques for determining the polishing endpoint included within thesecond category, monitor various operating conditions of the polishingapparatus and indicate an endpoint when one or more of the operatingconditions abruptly changes. An example of such a condition is thecoefficient of friction at the interface of the polishing pad and thesubstrate. When a metal layer is being polished to expose an underlyingdielectric layer, for example, the coefficient of friction will changewhen the dielectric layer is exposed. As the coefficient of frictionchanges, the torque necessary to provide the desired polishing pad speedalso changes. By monitoring this change such as by monitoring thepolishing motor current, endpoint may be detected. However, thecoefficient of friction is a function of the slurry composition, the padcondition, the load of the substrate on the pad, and the surfacecondition of the substrate. In addition, the pad condition and theslurry composition at the pad-substrate interface change as thesubstrate is being polished. Moreover, electrical noise may distort thecharacteristic being measured. Such effects may mask the exposure of theunderlying dielectric layer (and removal of the bulk of the metal film),and may cause endpoint to be indicated at the incorrect time.Additionally, using this technique, the endpoint detection will workonly if polishing is used to expose an underlying material having africtional attribute different than that of the material being removed.

Another technique for determining endpoint included within the secondcategory involves monitoring the current supplied to each of thepolishing motors such as the motor which rotates the polishing pad or amotor which may be used to rotate the substrate being polished. Usingthis technique, a determination that endpoint has been achieved, may bemade when a pre-determined total amperage is reached. Like the othertechniques within the second category of endpointing techniques, thistechnique also does not directly monitor physical activity occurring onthe surface being polished, during the polishing operation.

Therefore, none of the available endpointing techniques described above,detects endpoint by directly monitoring the amount of film beingremoved, or other physical changes occurring on the surface beingpolished, without interrupting the polishing process. As such, none ofthe known techniques for determining endpoint, do so by actuallysampling the surface during the CMP operation, and detecting that thebulk of the film being polished, is physically removed from the surface.It can be understood, then, that an endpointing method which activelymonitors the reaction occurring on the polishing surface, is desirablein the art of CMP.

For the aspect of CMP directed to forming conductive lines withintrenches or the like using damascene techniques, endpoint is achievedwhen the bulk of the conductive material is removed from over the uppersurface of the subjacent layer, but remains within the trenches formedin the subjacent layer, to produce a planar surface. At this point, itis desirable to terminate the polishing process. Since the polishingslurry includes chemical components which are reactive towards theconductive material being polished, the effluent slurry includescomponents of the conductive material being removed by polishing. Assuch, when endpoint is achieved, the composition of the effluent slurrychanges as the concentration of the conductive material within theeffluent slurry, drops.

It can be seen that there is a need to provide a method for detectingendpoint at this point in order to terminate or otherwise adjust thepolishing operation to avoid further undesired polishing.

SUMMARY OF THE INVENTION

The present invention relates to an endpoint detection method for use inconjunction with a chemical mechanical polishing operation. A metalcontaining film is polished using an acidic slurry. A sample of theeffluent slurry is directed to a vessel in which an electrochemical cellis formed and in which the sample serves as a liquid phase workingelectrode within the electrochemical cell. The electrochemical cellincludes a solid electrolyte solution chosen to allow for chemicalinteraction between the liquid phase working electrode and a referenceelectrode.

Half-cell reactions occur within the electrodes of the electrochemicalcell, and the difference in chemical potential between the workingelectrode and the reference electrode causes the diffusion of ionsacross the electrolyte. At equilibrium, the chemical potential gradientis balanced by an electric potential gradient and is indicated by astable open circuit electromotive force (emf) across the electrolyte.This emf is measured by lead wires placed in each of the electrodes, asa function of time. A signal is developed reflecting the electricpotential difference in time, and endpoint is detected responsive tochanges in the measured electric potential signal. When endpoint isdetected, the polishing operation may be terminated or otherwiseadjusted.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view showing metal and barrier layer filmsincluded within trenches formed in the substrate to be polished;

FIG. 2 is a cross-sectional view showing the structure as in FIG. 1,after the bulk of the metal film has been removed by polishing;

FIG. 3 is a cross-sectional view showing the structure as in FIG. 2,after the bulk of the barrier layer film has also been removed bypolishing;

FIG. 4 is a cross-sectional view of another exemplary embodiment of ametal film structure ready for polishing;

FIG. 5 is a cross-sectional view showing the structure as in FIG. 4,after the bulk of the metal film has been removed by polishing;

FIG. 6 is a side view of a CMP apparatus showing an effluent slurry; and

FIG. 7 is a representative view of an electrochemical cell which uses asample of the effluent slurry as a working electrode.

DETAILED DESCRIPTION OF THE INVENTION

As discussed above, chemical mechanical polishing may be used for a widerange of polishing operations. One common example of such a polishingoperation is the removal of a conductive film by means of polishing. Theremoval rate of the conductive film during a polishing operation dependson a number of factors such as the condition of the polishing pad, thecomposition and flow rate of the polishing slurry used, the powerprovided by the polishing motor, the film itself, the force applied tourge the polishing surface against the pad, the speed of rotation of thepolishing pad, and the speed of rotation of the substrate beingpolished. Because a number of the preceding factors may change during apolishing operation, it is generally not advisable to attempt toterminate a polishing operation based simply upon polishing time.

The present invention is directed to detecting endpoint during thepolishing of a metal-containing film. The metal-containing film may be ametal film commonly used as an interconnect material such as aluminum,copper, tungsten, or their alloys, or it may be a film commonly used asa barrier material such as tungsten silicide, titanium, titaniumnitride, titanium silicide, tantalum, tantalum nitride, tantalumsilicide, or other refractory metals. The film is removed by polishing.Endpoint is determined according to the present invention when the bulkof the metal-containing film, or the bulk of an individualmetal-containing film within a stack of films, is substantially removed.The bulk, metal-containing film being endpointed may be formed over aplanar substrate, or it may include portions additionally formed withinopenings formed in the underlying substrate.

A common application for chemical mechanical polishing is the removal ofportions of a bulk deposited film by polishing using damasceneprocessing techniques. In damascene, or dual damascene processingtechniques, an opening is formed within a substrate surface. A bulk filmis then deposited over the surface of the substrate and within theopenings. The polishing operation is used to remove the bulk of thedeposited film from over the plane of the substrate surface, whilemaintaining the deposited material within the openings, thereby forminga planarized surface including portions of the deposited material andthe substrate surface. When the deposited material is a conductivematerial such as metal, a wiring pattern may be formed of the conductivematerial which remains within the openings and forms conductive linesaccording to the above damascene processing techniques. FIG. 1 is across-sectional view showing a typical damascene structure ready forpolishing. In the exemplary embodiment shown in FIG. 1, the conductivefilm used in damascene processing comprises a composite film formed of abulk metal film over a barrier layer film. In other exemplaryembodiments, the barrier layer film may not be needed and the bulk metalfilm may be formed directly over the surface and within the openings.

In FIG. 1, subjacent layer 2 includes upper surface 10 and trenches 4which extend down from upper surface 10. Upper surface 10 is essentiallyplanar. In an exemplary embodiment, subjacent layer 2 may be aninsulating film formed over a semiconductor substrate such as a siliconwafer, or subjacent layer 2 may represent the semiconductor substrateitself. Trenches 4 may be formed within subjacent layer 2 by etching orother commonly available processing techniques. In an exemplaryembodiment, the trenches may be multi-tiered trenches (not shown) suchas used in dual damascene processing. The trenches 4 formed withinsubjacent layer 2, may alternatively be grooves or other openings suchas via openings. Hereinafter, these openings will be referred tocollectively as “trenches”.

Barrier layer film 6 is formed within trenches 4 and over upper surface10. Conductive material 8 is a film formed over barrier layer 6 andincludes top surface 14. In various exemplary embodiments, the barrierlayer film may be titanium, tantalum, tungsten, titanium nitride,titanium silicide, tantalum nitride, tantalum silicide, tungstensilicide or other refractory metals. Barrier layer film 6 may be formedusing any suitable conventional method. Conductive film 8 which fillstrenches 4 and is formed over top surface 12 of barrier layer film 6,will commonly be a metal such as aluminum, copper, tungsten, or theiralloys, but any metal-containing film used in such a damasceneapplication, may be polished and “endpointed” according to the presentinvention. During the chemical mechanical polishing operation, thickness20 of the bulk portion of conductive film 8 is reduced. Alternativelyillustrated, the uppermost surface of the substrate to be polished, isshown on grid 100 as position 114 which corresponds to top surface 14prior to polishing.

It should be pointed out that the figures are not drawn to scale.Rather, the features have been arbitrarily expanded or reduced forclarity, as is customary. For example, the trenches, as illustrated,suggest a greater depth and width than in practice. Also, the trenchesmay be spaced much farther apart than as illustrated. As such, when abulk film is formed over a surface containing trenches and also withinthe trenches, it should be understood that the amount of the filmincluded within the trenches is negligible when compared to the entiretyof the originally formed film.

FIGS. 1-3 illustrate a planarization process involving the sequentialremoval of films using a CMP operation. FIGS. 4 and 5 show a sequence ofprocessing steps used to remove and planarize another exemplaryembodiment of a film structure. The CMP operation used to remove thefilms by polishing, may be any suitable CMP process such as available inthe art. Any suitable CMP apparatus may be used. Likewise, the operatingconditions or parameters of the CMP operation and the settings of theCMP apparatus may be chosen as suitable for the particular CMP operationbeing performed. Parameters may include the speed at which the polishingpad is rotated, the speed at which the substrate is rotated, the flowrate and composition of the polishing slurry being supplied to thepolishing operation, the roughness of the polishing pad surface, thedownward force applied to urge the substrate being polished against thepolishing pad, and so forth.

As the film structure shown in FIG. 1 is subjected to a CMP operation,it can be seen that top surface 14 of conductive film 8, will bereceded. Now turning to FIG. 2, it can be seen that substantially all ofconductive film 8 which was formed over upper surface 10, has beenremoved by polishing. Conductive film 8 remains only in trenches 4, andbarrier layer film 6, including top surface 12, is exposed. As such,FIG. 2 represents the “endpoint” of the polishing operation for removingthe bulk portion of conductive film 8. Once again, it is stressed thatthe features of the trenches 4 have been expanded to show the structureformed within the trenches 4 and, as such, the trenches are shown inmuch closer proximity than in practice. It should be further understoodthat, relative to the overall surface of the substrate being polished,the surface area including the exposed surfaces 9 of the conductive film8 which lies within trenches 4, is very small in comparison to theexposed area of upper surface 12 of exposed barrier layer film 6 andthat the amount of conductive film 8, which remains in trenches 4, isnegligible when compared to originally formed conductive film 8.Position 112 on grid 100 corresponds to top surface 12 and indicatesthat the uppermost surface of the substrate being polished, has beenreceded with respect to the original uppermost surface indicated byposition 114 of grid 100, and the top surface 14 of originally formedconductive film 8, as indicated by the ghost pattern.

Now turning to FIG. 3, the CMP operation continues, as above, andremoves barrier layer film 6 from over upper surface 10. Upper surface10 of subjacent layer 2 is now exposed. Trenches 4 include sections ofbarrier layer film 6 disposed peripherally around the trench, andportions of conductive film 8. It can be seen that top surface 15 of thestructure formed within the trenches 4, is substantially planar withupper surface 10 of subjacent layer 2. FIG. 3 represents the endpoint ofthe polishing operation used to remove barrier film 12, and thereforethe second endpoint in the polishing process sequence. FIGS. 2 and 3therefore represent the two endpoints achieved in a sequential polishingsequence used to remove two films from over substrate, while leavingportions of each of the films within a trench formed within thesubstrate. Position 110 on grid 100 corresponds to upper surface 10 andshows the continued recession of the uppermost surface 10 of thesubstrate.

FIG. 4 shows another exemplary embodiment wherein a subjacent layer 52includes trenches 54 and upper surface 60. Conductive film 58 is asingle metal-containing film which has a top surface 64, and is formedover top surface 60 and within trenches 54. A suitable CMP polishingoperation may be used to remove conductive film 58 from over uppersurface 60 of subjacent layer 52, and which leaves portions ofconductive film 58 within trenches 54. It can be seen, in FIG. 5, thatportions of conductive film 58 have been substantially removed exposingupper surface 60 of subjacent layer 52. The resulting trench structuresinclude top surface 55 which is essentially coplanar with upper surface60 of subjacent layer 52. FIG. 5, therefore, represents the endpoint forthe polishing operation of conductive film 58, for a process where it isdesired to produce a planarized substrate surface having conductivelines formed within trenches formed therein, such as in damasceneprocessing.

It should be understood that the endpointing detection method of thepresent invention may be adapted to detect the endpoint condition shownin FIG. 2, FIG. 3, or FIG. 5. It should be further understood that theendpointing technique of the present invention finds much broaderapplication in the field of CMP. For example, a bulk, metal containingfilm, or a stack of bulk, metal containing films may be removed bypolishing, from over a substrate surface which is essentially planar.According to other exemplary embodiments, the metal containing filmswhich are being removed by polishing, and which may be endpointedaccording to the process of the present invention, may be formed oversubstrates having various underlying features.

FIG. 6 is a side view showing a substrate being polished using anexemplary embodiment of a CMP apparatus. Substrate 22 includes surface24 which is being polished. Surface 24 which is being polished, maycorrespond to any of the substrate surfaces shown in FIGS. 1-5, duringvarious stages of the polishing process. Substrate 22 which may be asemiconductor wafer on which integrated circuit devices are formed, asin the preferred embodiment, is held by carrier 26 which is securelyattached to shaft 28. The CMP operation includes a mechanical aspectwhich includes a force F which urges the shaft, carrier, and substratealong direction 29 to force the substrate surface 24 against thepolishing pad surface 20. In an exemplary embodiment, shaft 28 mayrotate about its axis thereby rotating carrier 26 and substrate 22,thereby mechanically moving surface 24 with respect to pad surface 20.Also in the exemplary embodiment, pad surface 20 may be rotated withrespect to the assembly including spindle shaft 28, carrier 26, andsubstrate 22. Pad surface 20 will be rotated about an axis other thanthe axis of shaft 28. The two relative motions provide a mechanicalaspect to the chemical mechanical polishing operation.

A liquid polishing slurry inlet stream 23 is directed to interface 27formed between polishing pad surface 20 and surface 24 of substrate 22which is being polished. Although surface 24 substantially contacts padsurface 20 to enable polishing, interface 27 has been expanded, in thefigure, to illustrate that polishing slurry is included at theinterface. Effluent slurry stream 25 exits the surface being polished.It should be understood that the above description of a CMP apparatus isintended to be exemplary only. Various other CMP apparatuses whichprovide such an effluent slurry stream, may be used alternatively.

The polishing slurry includes abrasives which aid in the mechanicalaspect of the polishing operation. The slurry may further includechemical aspects. When a metal containing film is being polished, theslurry will include an acidic component. The acid within the slurry aidsin attacking and removing the metal containing material being polished,by way of chemical reaction. The slurry solution may be chosen toinclude any suitable acid depending on the polishing operation and thecomposition of the metal containing film being polished. Sulfuric acid,acetic acid, propionic acid, and phosphoric acid are examples of acidscommonly included in polishing slurries. During the polishing operation,the metal within the metal containing film is oxidized by the acidaccording to the following equations:

M_(film)+O→MO_(x,)removed   (2)

It can be therefore seen that the effluent slurry stream 25 containsproducts of the above reactions, including oxidized metal (MO_(x))formed from the metal in the metal containing film which is beingremoved by polishing. In various exemplary embodiments, metal “M” mayrepresent aluminum, copper, tungsten, titanium, tantalum or othermaterials. CMP is a continuous operation, and inlet slurry stream 23 isfed continuously to the operation. Consequentially, effluent slurrystream 25 is continuously produced and may be withdrawn from theoperation using conventional methods. During the polishing of a metalcontaining film using an acidic slurry, the continuously producedeffluent slurry stream 25 will continue to contain a concentration ofoxidized metal, as above.

When an endpoint condition is attained, as shown in FIGS. 2, 3, and 5,there is very little oxidized metal being produced. As such, theconcentration of oxidized metal within effluent slurry stream 25 dropssignificantly. The present invention is directed to detecting endpointby detecting a change in the components within the effluent slurrystream 25 as shown in FIG. 6. More specifically, the present inventionis directed to detecting an endpoint condition by noting a change in theactivity of the components within effluent slurry stream 25.

A sample of effluent slurry stream 25 is withdrawn from the polishingoperation, then introduced into an electrochemical cell, in which thesample of the effluent slurry stream functions as a working electrode.FIG. 7 shows effluent slurry stream 25 being delivered by way ofdelivery means 30 to electrochemical cell 31. The delivery of slurrystream 25 to electrochemical cell 31 may be continuous or intermittent.Electrochemical cell 31 includes a vessel 37 for retaining a sample ofeffluent slurry stream 25 which is provided to the electrochemical cell31. Conventional means for withdrawing effluent slurry stream 25 fromthe site of the polishing operation, specifically from the interfaceformed between the polishing pad and the surface being polished as shownin FIG. 6, may be used to deliver a sample of effluent slurry stream 25to electrochemical cell 31.

Electrochemical cell 31 employs a solid electrolyte 44 separatingworking electrode 33 from reference electrode 34 and enabling thetransfer of anions across solid electrolyte 44. In the preferredembodiment, a solid yttria stabilized zirconia solution (YSZ) may beused as solid electrolyte 44. YSZ is an ideal solid electrolyte whichselectively permits only O⁻² ions to diffuse through it. In an exemplaryembodiment in which electrochemical cell 31 is designed to allow thehalf-cell reactions in the reference 34 and working 33 electrodes toinvolve liberation of O⁻² at one electrode compartment and thesimultaneous consumption of O⁻² at the other electrode compartment, thetransfer of O⁻² ions across solid YSZ electrolyte 44 occurs. Thisexemplary embodiment represents an oxide-type solid-stateelectrochemical cell.

Since the metals which are commonly used in the semiconductormanufacturing industry (W, Ti, Ta, Cu, Al) form stable metal oxides, anoxide-type solid-state electrochemical cell as above, is well suited foran endpoint detection system using electrochemical techniques.

Liquid-phase working electrode 33 consists of a sample of the effluentslurry in conjunction with a metal oxide which is used to initiate thehalf-cell reaction in working electrode 33 of the electrochemical cell31. Working electrode 33 is contained within vessel 37. Metal oxidesolids 38 may be added within vessel 37 of electrochemical cell 31 toserve as the metal oxide source used to initiate this reaction withinthe working electrode. Reference electrode 34 is a solid phaseheterogeneous mixture of a pure metal, and its corresponding metaloxide. Solid phase reference electrode 34 may be in powder form. Themetal included in reference electrode 34 will preferably be the samemetal which is being polished and which is therefore included ineffluent slurry stream 25. This ensures execution of complementaryhalf-cell reactions in both working electrode 33 and reference electrode34 of electrochemical cell 31. The metal oxides added to both theworking electrode (solids 38), and included in reference electrode 34,do not adversely effect the working of electrochemical cell 31. Theinvolvement of the metal oxide occurs only during the half-cellreactions; their effect is nullified during the overall cell reaction.The electrochemical cell may therefore be described as:

Working Electrode || Electrolyte || Reference Electrode

M_(slurry), MO_(x) || YSZ || M_(pure,)MO_(x)

Within vessel 37, the sample of effluent slurry forms liquid-phaseworking electrode 33 of electrochemical cell 31. Outlet port 42 providesan outlet for liquid-phase working electrode 33. According to theillustrated embodiment, a steady state condition may be achieved after aliquid level 46 is achieved in vessel 37, at which point the amount ofslurry sample being introduced by effluent slurry stream 25 equals theamount of slurry sample being removed through outlet stream 43. In thisembodiment, it can be seen that liquid-phase working electrode 33 is adynamically changing solution. In another embodiment, outlet port 42 maynot be used to provide a dynamically changing effluent slurry sample,and discrete samples of effluent slurry stream 25 may be periodicallyintroduced into electrochemical cell 31 to form working electrode 33,then dispensed. Lead wire 35 is provided within working electrode 33 toprovide electrical contact to external electronic circuitry throughterminal 40.

In an exemplary embodiment, the effluent slurry solution may be heatedusing conventional means, either as it is being delivered to theelectrochemical cell, or once within the electrochemical cell. In thepreferred embodiment, the effluent slurry stream may be heated to atemperature within the range of 300° C. to 400° C., but othertemperatures within the range of 200° C. to 600° C. may be usedalternatively.

Solid electrolyte 44 separates liquid-phase working electrode 33 fromsolid phase reference electrode 34, within electrochemical cell 31.According to the exemplary embodiment, reference electrode 34 isphysically contained within solid electrolyte 44 which is also incontact with liquid-phase working electrode 33. In an exemplaryembodiment, solid electrolyte 44 shares a conterminous surface withworking electrode 33. Within reference electrode 34 is lead wire 36which is also coupled to terminal 39 for external circuitry.

Solid electrolyte solution 44 is chosen to allow for the exchange ofions between reference electrode 34 and working electrode 33. In apreferred embodiment, solid electrolyte solution 44 is chosen to allowfor the interchange of oxygen (O⁻²) ions between liquid-phase workingelectrode 33 and solid-phase reference electrode 34. In the exemplaryembodiment shown, solid electrolyte solution 34 may take on acylindrical shape which contains reference electrode 34. It should beunderstood, however, that the shape of the solid electrolyte solution isnot intended to be limited to the cylindrical shape shown in theexemplary embodiment.

In the preferred embodiment where the solid electrolyte 44 may be asolid solution of yttria stabilized zirconia (YSZ), the YSZ may comprise2 to 12 weight percent of Y₂o₃ within Zr₂o₃. The solid YSZ is selectiveto O⁻² ions. The O⁻² ions are transferred between reference electrode 34and liquid phase working electrode 33 as the following two half-callreactions take place within the electrochemical cell: $\begin{matrix}\left. {{{Reference}\quad {Electrode}\quad (34)\quad M_{pure}} + {\frac{x}{2}0^{- 2}}}\Rightarrow{{MO}_{x} + {xe}^{-}} \right. & (3) \\{\left. {{{Working}\quad {Electrode}\quad (33)\quad {MO}_{x}} + {xe}^{-}}\Rightarrow{M_{slurry} + {\frac{x}{2}0^{- 2}}} \right.{{{Overall}\quad {Cell}\quad {Reaction}\quad M_{pure}} = M_{slurry}}} & (4)\end{matrix}$

Within the electrochemical cell, the above reactions occursimultaneously, resulting in a difference in chemical potential (molarconcentration) of the O⁻² ions across the electrolytes which separatethe half-cells. This difference in chemical potential across theelectrolytes is afforded by O⁻² ions diffusing from the oxidized metalwithin the slurry which forms liquid phase working electrode 33, to themetal contained within reference electrode 34.

The O⁻² ions diffuse across the electrolyte such that the generation ofO⁻² in one half-cell reaction is balanced by the consumption of O⁻² ionsin the corresponding half-cell reaction. This creates an excess (due toO³¹ ² consumption and as shown in the reference electrode 34) orscarcity (due to O⁻² generation and as shown in the working electrode33) of electrons (e⁻) in the corresponding electrode as each electrodetends towards electroneutrality. This results in a net positive or netnegative potential developed on either of the lead wires 35 or 36. Thedifference in electric potential between lead wires 35 and 36 is calledthe open circuit potential or electromotive force (emf).

At equilibrium, the chemical potential gradient caused by the diffusionof O⁻² ions across the electrolytes, is balanced by the electricpotential gradient and is indicated by a stable open circuitelectromotive force (emf) across the electrolyte. This electricpotential gradient can be measured across terminals 39 and 40 usingconventional electronic circuitry 50, or other means. This gradient, ordifference in electric potential, may be read in volts or millivolts.The electric potential difference may be read essentially continuouslyor periodically, at a suitable frequency.

The open circuit emf is directly related to the chemical activity of thepolished component in the slurry by the Nernst equation, which governsthe electric potential of the electrodes of each half-cell and may beexpressed as:

E=E°−(RT/zF)·lnK   (5)

- or -

(6) E=E°−(RT/zF)·ln(a[RED]/a[OX])

In the Nernst equation, z represents the valance of the species beingreduced, F represents the Faraday constant, E represents potential ofthe electrode of the half-cell reaction, E° is the standard reversiblepotential of the electrode, R represents the universal Gas constant, Trepresents temperature, K is the equilibrium constant, a[RED] is theactivity coefficient of the reduced species of the half cell reaction,and a[OX] is the activity coefficient of the oxidized species of thehalf cell reaction. In solution, the activity coefficients are generallytaken as the molar concentrations. For the reference electrode, theactivity of the metal is a[OX]=1. Thus, equation (6) simplifies asfollows:

ZF(E−E°)=−RT In a[RED]  (7)

The quantities of the left hand side of equation (7) are known or may bemeasured. When the metal activity, a[RED] in the slurry changes, it willbe reflected as a change in the open circuit emf (E−E°) which may bemeasured using conventional means. Furthermore, the activity a[RED] ofthe metal species can be calculated using the measured emf.

When an endpoint condition is attained, the concentration of theoxidized metal species being produced as a result of the polishingoperation and which is included within the effluent slurry, dropssignificantly. As the concentration of the oxidized metal species drops,so does the activity a[RED], resulting in a new equilibrium conditionfor the half-cell reactions and, therefore, a change in the measuredopen circuit emf. A conventional electronic circuit 50, or other meansmay be connected across terminals 39 and 40 to measure the emf acrossthe open circuit which changes as a result of the changed compositionand activity of the metal species within the effluent slurry solution.

A signal may be developed from the measured open circuit emf. Anysuitable conventional method may be used to measure the open circuit emfand to display a signal of the measured value over time, eitherdigitally, graphically or using other electronic means. As such,conventional means used to measure an electric potential gradient acrossterminals 39 and 40 (the open circuit emf), can be used to detect achange in the component activity within the effluent slurry stream, suchas which occurs when endpoint is attained.

Reactions taken in response to the determination that endpoint conditionhas been attained, may be taken when a single abrupt change in theelectric potential signal is detected, or in response to a steady statecondition being achieved by the electric potential signal after achange. The time at which to respond to a change in the emf signal, maybe determined by the polishing operation being performed, and willreflect the amount of “overetch” desired. Once a determination ofendpoint has been made, the reaction taken to adjust the polishingoperation may be a manual one, or it may include an electronic circuitproviding a real time, feed-forward signal to automatically adjust thepolishing operation. Conventional electronic circuitry available in theart may be used to provide a signal to the polishing apparatus inresponse to endpoint being detected.

The sampling frequency of the electrical potential measurement takenbetween terminals 39 and 40 will vary based on the polishing operation.For example, in the case where 10,000 angstroms of a copper film isbeing removed by polishing, the expected time for endpoint to beattained, may range from 2 to 6 minutes depending on the polishingconditions used. For such a polishing operation, a sampling frequency of5 to 10 seconds may be used. In other exemplary embodiments, thesampling frequency may vary based on the film being polished, thethickness of the film being polished, and the polishing conditions. Inanother exemplary embodiment the electric potential difference may bemeasured virtually continuously.

In response to an endpoint condition being detected, the entirepolishing process may be terminated, or the polishing process may beotherwise adjusted. The adjustment of the polishing process may includeadjusting any of the various parameters and settings which affect theoperating conditions. For example, a buffing operation may be initiated,or a further polishing operation may be commenced to polish underlyingfeatures. In any event, by accurately determining the endpointcondition, further unwanted polishing is avoided and undesired effectssuch as dishing within the underlying features, is prevented.

Now referring again to FIGS. 2 and 3, the first endpoint condition inthe polishing process, may be attained and detected at the point of thepolishing process represented by FIG. 2. At that point, the polishingprocess may be adjusted to produce a polishing operation directed topolishing barrier layer film 6. Likewise, a second endpoint condition inthe polishing process, may be attained and detected, at the point of thepolishing process represented by FIG. 3. After the removal of the bulkof barrier layer film 6 from over upper surface 10, and as indicated bya change in the electric potential measured across terminals 39 and 40as a result of the change of concentration of polished metal species inthe effluent slurry solution, a number of actions may be taken. Forexample, the polishing process may be adjusted and allowed to continue,the process sequence may be terminated, or a buffing operation may beinitiated.

The preceding merely illustrates the principles of the invention. Itwill thus be appreciated that those skilled in the art will be able todevise various arrangements which, although not explicitly described orshown herein, embody the principles of the invention and are includedwithin its spirit and scope. Furthermore, all examples and conditionallanguage recited herein are principally intended expressly to be onlyfor pedagogical purposes and to aid the reader in understanding theprinciples of the invention and the concepts contributed by theinventors to furthering the art, and are to be construed as beingwithout limitation to such specifically recited examples and conditions.Moreover, all statements herein reciting principals, aspects, andembodiments of the invention, as well as specific examples thereof, areintended to encompass both structural and functional equivalentsthereof. Additionally, it is intended that such equivalents include bothcurrently known equivalents such as equivalents developed in the future,i.e., any elements developed that perform the same function, regardlessof structure. The scope of the present invention, therefore, is notintended to be limited to the exemplary embodiments shown and describedherein. Rather, the scope and spirit of the present invention isembodied by the appended claims.

What is claimed:
 1. A method for detecting endpoint in a chemicalmechanical polishing operation, comprising the steps of: (a) polishing ametal-containing film using a chemical mechanical polishing operationincluding an acidic slurry; (b) withdrawing an effluent slurry streamfrom said polishing operation; (c) directing a sample of said streaminto a sampling vessel; (d) providing a solid electrolyte coupled to areference electrode within said sampling vessel and wherein said streamsample forms a liquid-phase working electrode contacting said solidelectrolyte and thereby forming an electrochemical cell, said solidelectrolyte selected to allow for chemical interaction between saidreference electrode and said working electrode; (e) measuring anelectric potential difference between said working electrode and saidreference electrode; and (f) adjusting said polishing operationresponsive to a change in said difference.
 2. The method as in claim 1,wherein said reference electrode includes metal and oxidized metal of ametal species contained in said metal-containing film.
 3. The method asin claim 1, in which said step (d) includes providing an outlet portwithin said sampling vessel and wherein said working electrode is formedof a dynamic stream sample.
 4. The method as in claim 1, furthercomprising heating said effluent slurry stream.
 5. The method as inclaim 4, wherein said heating comprises heating to a temperature withina range of 200° C. to 600° C.
 6. The method as in claim 1, in which saidstep (d) includes providing said solid electrolyte having a surfacebeing conterminous with said working electrode and a further surfacebeing conterminous with said reference electrode, and which selectivelyallows for an interchange of oxygen ions between said working electrodeand said reference electrode.
 7. The method as in claim 1, in which saidstep (d) includes providing said solid electrolyte adapted for directexchange of oxygen ions between said working electrode and saidreference electrode, said solid electrolyte comprising a solid yttriastabilized zirconia solution.
 8. The method as in claim 1, wherein saidelectric potential difference is produced by a diffusion of ionsresulting from components of said reference electrode being oxidized andcomponents of said working electrode being reduced.
 9. The method as inclaim 1, in which said step (a) of polishing includes oxidizing metal ofsaid metal-containing film, and in which said effluent slurry thereforeincludes oxidized metal.
 10. The method as in claim 1, in which saidstep (a) includes polishing a film of one of Ti, TiN, Ta, and TaN. 11.The method as in claim 1, further comprising step (e1) developing asignal based upon said difference, and wherein said step (f) comprisesadjusting said polishing operation responsive to a change in saidsignal.
 12. The method as in claim 1, further comprising the step b1) ofproviding a solid metal oxide of a metal species contained in saidmetal-containing film, in a portion of said sampling vessel adapted toretain said working electrode.
 13. The method as in claim 1, in whichsaid step (a) includes polishing a film of one of aluminum, copper, andtungsten.
 14. The method as in claim 1, wherein said step (e) comprisesmeasuring an open circuit electromotive force of an open circuitincluding said reference electrode and said working electrode.
 15. Themethod of claim 1, in which said step (a) includes providing asemiconductor substrate having said metal-containing film disposed overa further metal-containing film, step (f) includes adjusting operatingconditions of said polishing operation to further polish said furthermetal-containing film; and further comprising step: (g) furtherpolishing said further metal-containing film.
 16. The method as in claim1, in which said step (a) includes providing a semiconductor substratehaving said metal-containing film disposed over a surface thereof. 17.The method as in claim 16, in which said step (a) further includes saidmetal-containing film being disposed within trenches formed within saidsurface, and in which said step (f) includes terminating said polishingoperation when said metal-containing film is substantially removed fromover said surface and substantially remains only within said trenches.18. The method as in claim 1, in which said step (a) includes polishinga refractory metal film.
 19. The method as in claim 1, furthercomprising step (el) providing an electronic circuit capable ofautomatically adjusting said polishing responsive to a change in saiddifference, and wherein said step (f) comprises automatically adjustingsaid polishing.
 20. The method as in claim 1, wherein step (f) comprisesterminating said polishing operation.
 21. The method as in claim 1,wherein step (f) comprises changing at least some operating conditionsof said polishing operation.